This invention relates to frequency measuring systems.
U.S. Pat. No. 4,112,358 of Ashida, issued Sept. 5, 1978, discloses a frequency measuring device involving the principle of period measurement with a counter and a vernier correction, where the vernier measures a fraction of one clock period. An integrating voltmeter reads fraction of period converted into said frequency resolution.
The counter-vernier principle will operate very well on stable frequencies with low phase noise but, in the case of phase noise, the periods of the first and last cycles which are used to compute the "vernier", or frequency correction, will be unstable.
In the present invention the "vernier" quantity is averaged by an analog low pass filter, the frequency characteristics of which may be selected as required, e.g. to reject interfering frequencies. A system according to the invention will produce a stable reading even with input signals affected by high phase noise.
A variation of the prior art counter-vernier principle, adopted for low frequency operation, is disclosed by U.S. Pat. No. 4,224,568 of Griner, issued Sept. 23, 1980. The system of that patent takes the sum of the full signals counted plus the fractional signal at the beginning of the sampling period and the fractional signal at the end of the sampling period to provide an approximation of the actual frequency being measured.
U.S. Pat. No. 4,107,600 of McMannis, issued Aug. 15, 1978, discloses a system based on period measurement which is converted into frequency. The system is applicable only to stable frequencies. An "adaptive" part is used to maintain constant resolution for low frequency measurements.